The first PWM amplifiers comprised triangular generators. The triangular signal from the triangular generator is typically compared to an input signal by a comparator forming a PWM signal on the output of said comparator. This is the typical textbook example when designing a switching amplifier. One such prior art amplifier is described in US-A-2004/0846281.
The triangular generator approach has several drawbacks. The modulator has typically not any feedback correction, leading to the derived PWM signal comprising the non-linearities of the triangular generator. If a control loop is applied to the amplifier it will be restricted in bandwidth and loop gain in order to comply to the Nyquist stability criteria. This results in high levels of distortion along with a high load dependency of the closed loop transfer function of the amplifier.
As an alternative to using a triangular generator, some prior art PWM amplifiers use instead a self-oscillating modulator. Self-oscillating modulators eliminate major disadvantages of the triangular generator pulse width modulation. This is due to the increased level of bandwidth and loop gain that these modulators possess because of less restrictions to the Nyquist stability criteria.
The modulator and control system in a switching amplifier defines parameters such as THD+n (Total Harmonic Distortion plus noise), Intermodulation distortion, closed loop −3 db bandwidth, step response capability, load dependency, load step response, output impedance etc.
One known way of achieving a self-oscillating modulator is to use a self-oscillating local loop modulator.
Self-oscillating local loop modulator- and control systems comprising only a local loop can be seen from several documents, such as U.S. Pat. No. 6,300,825. In this context the term “local loop” is to be understood as a feedback loop where the feedback signal is derived before the output filter.
These types of modulators lacks the controlling capability of the demodulation filter in the power converter resulting in high levels of e.g. output impedance and distortion defined mainly by the non-linearities of the output filter component.
Another known way of achieving a self-oscillating modulator is to use a self-oscillating global loop modulator. In this context the term “global loop” is to be understood as a feedback loop where the feedback signal is derived after the output filter.
Self-oscillation global loop modulation comprising modulators in a single global loop has an advantage on lower output impedance, better step responses and has the potential for better modulator linearity. Such a modulator can be seen from the patent application “Global loop integrating modulator” international publication number WO-A-2004/100356. These types of modulators have the disadvantage that the sensitivity is in some cases a trade off with the modulator linearity. In order to obtain higher sensitivity the global loop can be cascaded as nested loops, resulting however at the same time in a compromise on load step- and transient stability capability.
Typically, ordinary non-oscillating global loop feedbacks, as disclosed in WO-A-01/71905, generally exhibit lower error suppression capability of an applied local modulator loop and at the same time reduces the loadstep and step response capabilities resulting in a trade-off between sensitivity of the global loop and stability, as compared to self oscillating control systems. The overall compromise will result in a system with high output impedance and low load step and input step capabilities.
Output residuals will be fed into the local loop modulator by the global loop resulting in modulator distortion. This type of system has also got a large switch frequency variation resulting in very low switching frequencies at high output amplitudes of the amplifier. Such a system has been described in U.S. Pat. No. 6,297,692.